Carey Roach, MD
Internal Medicine and Pediatrics-Combined
There has been a lot of confusion regarding the accuracy of the rapid antigen Covid 19 test versus the PCR, and when to use them. The rapid test, or the Covid 19 Rapid Antigen Test, is typically the rapid test that has results in 15 minutes. It is the one that detects antigen (protein specific to that virus) showing two lines if positive, like a home pregnancy test. This is the test the U.S. government is making available to make testing as accessible as possible. It works by having a dye on a special membrane strip, and the dye binds to antibodies in the membrane that then bind to Covid 19 antigen proteins, if present. The combination of the three agents converts the dye to a visible form, showing up as a line on the test strip.
The Covid 19 PCR (Polymerase Chain Reaction) send-out test is more accurate, detecting the presence of the virus’ actual RNA, a fingerprint more specific to that virus, taking anywhere from 4 hours to 3 days to return with a result. More testing sites can run the PCR within a few hours, getting the results back later that day. Often the PCR is also referred to as the Nucleic Acid Amplification Test (NAAT) or RT-PCR (Real-Time Reverse Transcriptase PCR) Test. I see RT-PCR mentioned especially in required testing by airlines or cruise ships, which is confusing, because "RT" can be misinterpreted as the "Rapid Test", and it is not.
For a nice explanation and graphic of PCR testing, follow this link: https://www.genome.gov/about-genomics/fact-sheets/Understanding-COVID-19-PCR-Testing
So which test should I do?
Let’s look at the Pros and Cons of each test, then I will discuss which test to use and when.
Rapid Antigen Test
Pros:
You know almost instantly if you are positive for Covid 19
Typically, if the rapid test is positive for Covid 19, then Covid 19 is typically present. Rarely are there false positives, but some of the more inferior tests may have false positives at the time.
Rapid tests are easier to obtain and can be stored at room temperature on a grocery store shelf or your house.
Rapid tests are more readily available at outpatient clinics
Cons:
The rapid test is not very sensitive, so if it is negative, you still have a high probability, 15-40% chance, that you have Covid 19. It does not detect the virus well.
It usually does not test positive if you are asymptomatic. Asymptomatic people typically do not shed as many viral particles as symptomatic, and a test that is not very sensitive is less likely to detect the presence of the virus.
It usually does not test positive if you test too soon, less than 5 days of symptoms.
PCR (Polymerase Chain Reaction)
Pros:
If it is positive for Covid 19, then Covid 19 is typically definitely present. Rarely are there false positives.
It is a more sensitive test, needing less viral concentration for detection, therefore detecting Covid 19 better than rapid testing when you are asymptomatic.
It can detect Covid 19 earlier than 5 days of symptoms.
If it is negative, you have a higher probability than with the rapid test that you do not have Covid 19. It's accuracy it typically in the 92-97% when tested at the right time in a disease course.
Cons:
It takes longer to get results, sometimes 3 days. But if you find a testing site that can get the results back the same day, 4-12 hours later, then that test is still very rapid.
It is more expensive than the rapid antigen test.
It is so sensitive that you might test positive for 60-90 days after initially testing positive, even though you would not be contagious after 8-10 days after exposure; 14 days at the most in rare circumstances. In fact, the CDC recommends no one retest for Covid 19 within 90 days after testing positive for Covid 19.
When to use a Rapid Antigen Test:
If you are symptomatic and need to know immediately if you have Covid and you do not have access to a PCR test in the next day, or it will take over one day for a PCR test results to return.
If you have immediate access to a rapid test and you have been symptomatic for 5 days or more.
When to use a PCR Test:
Whenever you have access to one. Pretty simple. The PCR test is that much better than rapid tests.
If you have a negative rapid antigen test.
Timing is very important.
Try to avoid testing with either type of testing when it has been less than 5 days since exposure, and you are without symptoms.
Try to avoid rapid testing until you have had 5 days of symptoms.
Try to avoid PCR testing until you have had at least 3 days of symptoms, although it is still preferable to wait until day 5 before testing.
Bottom Line: Try to use the PCR test first-line and use the rapid antigen test only if you must.
The lack of sensitivity of the rapid antigen test is an inherent characteristic of the test, and that is not a bad thing if you know the limitations of the test and do not rely on it as your sole source of information.
Too many times I have seen patients and even friends/acquaintances put too much stock in the rapid antigen test, believing erroneously that they do not have Covid 19 if the rapid test is negative, and without confirmatory PCR testing. They would declare that they do not have Covid 19, sometimes while coughing and blowing their snotty nose. Even as a doctor, that’s still kind of gross. By the way, it was still contagious, whatever it was. Thanks for sharing.
Real-World Scenarios for Testing
Let's look at Covid 19 testing scenarios that I see every week.
I've been exposed to Covid 19 and I get a rapid test, and it is positive: I'm positive.
I've been exposed to Covid 19 and I get a rapid test, and it is negative: Am I negative? Heck, no! (Edited so my elder board at church does not call me.) Actually I do not know yet. I need to follow up with a PCR test 5 or more days after exposure.
I've been exposed and get the rapid test 5 or more days after exposure, and it is negative: Nope, I still do not know if I have Covid 19 or not, so I must follow up with a PCR test.
I got a PCR test and it is positive: I'm positive.
I got a PCR test 5 or more days after exposure or symptoms began and I was negative, I am most likely negative. What do I mean that I am "most likely negative?" There is still a slight chance I have a false negative PCR test, so If I have symptoms, I should quarantine the remaining 5 days, just in case, if possible.
Limiting testing avoids tying up appointments and testing times for those who more urgently need them.
Another issue to consider is, given the limitations of the rapid test, only getting a rapid test at a clinic or in a Covid testing line, while not getting a PCR, ties up an opportunity for someone else to get tested. Also, some clinics, including mine, have had shortages of Covid 19 rapid tests. If you use a rapid test, try using one over the counter or provided to your household by the health department or CDC.
Either way, if you need to get tested, test at the right time, and make it count, preferably with a PCR.
For more details on quarantining, see my previous blog link below: “Quarantine Quandary: How Long Do I Quarantine For Covid? How Did We Get Here?”
You are entering the Nerd Zone past this point.
Now for a more in-depth discussion of my rationale above, please read further. If you have enough information, you may stop here, and get on with your life, hopefully with a few more things to consider; but if you want further discussion, please continue!
Limitations of Rapid Antigen Testing.
The rapid Covid 19 test is a good test, but it, like other rapid antigen tests (such as influenza or streptococcal rapid tests) requires a large concentration of viral antigen to detect the virus. Antigens are proteins in or on the virus that trigger an immune reaction.
Rapid testing is then vulnerable to several variables that may limit the concentration of viral antigen present for testing:
· The swab was not inserted far enough up the nose to obtain enough sample.
· The wrong type of material swab was used.
· The test was done too early or too late in the time course of the viral infection.
· The level of severity of symptoms at the time of testing.
· The presence of blood in the nasal cavity when swabbed.
It is also vulnerable to some quality-assurance variables:
· The test shelf-life had expired.
· The test came from a box of tests that were defective and were not tested beforehand with a known control solution before using.
Some analogies of the error of relying on rapid testing.
Often there is not enough viral material present for testing.
Testing with a rapid antigen test, and not detecting Covid 19, is like trying to take a picture of a dimly lit lion on a safari with Polaroid self-developing film and not seeing it on film, although the lion is there. It would be foolish to assume the lion was not there just because it was not on the film. It would be more foolish to declare to your friends there is no lion and go about your evening until moments later your evening is interrupted with your own mauling. I bet that will show up on film.
Testing with a PCR test would then be like taking a picture of the same dimly-lit lion with a more advanced digital camera that can detect low light, confirming the presence of the lion.
The safest approach would be to assume the lion was there, taking evasive action (kind of a reverse quarantine), and get inside until you know if it is there or not. Do not rely only on your detection method.
Often, the timing of the test is too early.
Timing of testing is important with both the rapid test and the PCR, but especially with the rapid test. In the same analogy, it would be better to wait for the sun to rise higher, illuminating the lion with more light, making it more likely that the Polaroid camera would capture the image.
Another example of the importance of timing is that a home pregnancy test, if tested too early in the pregnancy, may test negative. You cannot assume you are not pregnant when you tested only two weeks after intercourse, then declaring “I’m not pregnant!” (In other words, you have been exposed, are “asymptomatic”, and have tested too early.) The home pregnancy tests are much more accurate after testing 4 to 6 weeks after conception when the beta hCG hormone is at a high enough concentration to be detected.
If someone were tested during the first few days of developing the viral infection, that person has fewer viral particles to be detected, and the less sensitive test would be even that much less likely to detect it. But if the person were to be tested toward the middle or latter part of the infection course when the viral particles are at their greatest, the test is more likely to detect the viral particles.
If all you have is the Rapid Antigen Test.
Having said all the above, if you have a rapid antigen test at home already or have access to one, use it if you have symptoms, and have had symptoms at least for 3 days, preferably 5 days. Always remember to get a PCR test if the rapid test is negative.
Remember not to be retested for Covid 19 during the 90 days after testing positive for Covid 19.
Always seek medical attention if you test positive for Covid 19, especially if you have symptoms.
Always quarantine as per the current guidelines if you are positive for Covid 19.
For information on how and when to quarantine, click on the link below:
Carey Roach, MD is Board Certified in both Adult Internal Medicine and Pediatrics-Combined. Translated, he specializes in generalities and, sometimes, freaky medical problems of people from birth until 116 years. After 116, you're on your own. He practices primary care medicine at the Barg Family Clinic in Little Rock, Arkansas. Currently, his hobbies include learning to kayak, finishing reading The Lord Of The Rings trilogy, dating his bride of 23 years and experiencing first-hand her excellent culinary skills. He also gets to share with her in teaching and mentoring soon-to-be married couples. When there is not a global pandemic, he loves to travel with his wife and kids. His views are his alone, although based on research and experience; and do not necessarily reflect the views of the management or staff of the Barg Family Clinic (although he thinks they should.)
As with any medical information, verify it with your physician before making major medical decisions that would affect your health or the health of others.
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